Title: STREAMLINE
Author: m.liberato
Last saved by: Amanda Hendrix
Company: Documint

FTS-NASA-Voice
Moderator: Trina Ray
04-27-07/6:00 pm CT
Confirmation # 3668884
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RAW TRANSCRIPT – NOT YET REVIEWED FOR CORRECTIONS BY CASSINI PERSONNEL

FTS-NASA-VOICE

Moderator: Trina Ray
April 27, 2007
6:00 pm CT

Man:	…the PowerPoint are tabled by numbers, right?

Coordinator:	Thank you all for standing by.

	At this time, the conference call is being recorded. If you have any objections, you may disconnect.

	Please go ahead.

Amanda Hendrix:	Thank you.

	Okay. Good morning, everyone. Good afternoon.

	This is Amanda Hendrix. I'll be hosting the CHARM telecon today.

	And I'm pleased to introduce today's speaker, Dr. John Spencer of the Southwest Research Institute in Boulder, Colorado.

	John works on Cassini's Composite Infrared Spectrometer or the CIRS instrument, mapping temperatures in Saturn's moons. And he's also extensively studied the Galilean moons of Jupiter using data from the Galileo's spacecraft, as well as the Hubble Space Telescope.

	He's also in the New Horizons Team, the spacecraft on route to Pluto with a spectacular Jupiter Flyby a few weeks ago.

	John is an expert on Jupiter's volcanic moon, Io, and so it seems like a natural transition for him to now be setting Saturn's volcanic moon, Enceladus.

	Today, John is going to tell us all about Enceladus, Cassini's discovery of volcanic activity, and the works that he and others have been doing to understand the sources and the facts of that activity.

	So, John, please go ahead and start.

John Spencer:	Okay. Thank you, Amanda.

	I guess we'll move straight onto the first real slide, Slide Number 2. And I'll just say a little bit about what we knew about Enceladus before Cassini got there, just to set the stage for all the amazing things that Cassini has found.

	So, we first landed (unintelligible) a long time ago back to the Voyager Flybys in 1980 and 1981. I was lucky enough to be at the Voyager II Flyby as a humble graduate student when I - we saw these pictures coming in and they really blew everyone away as to what a strange world this was.

	As most of Saturn's small icy moons are pretty heavily cratered, maybe have a few plains regions, but Enceladus is intensely fractured, has these linear features all over it. What craters it does have are heavily modified. You can see craters here that looked like they've almost just sort of melted back into the surface.

	And Voyager also showed that Enceladus has a very high albedo. Its surface is almost pure white. It's the brightest object in the Solar System. And so that's telling me there's something very unusual happening there. But we didn't really know what back then.

	The next slide, Slide 3, gives - say something about what we thought was going on.

	Voyager of course had discovered of the volcanoes on Io. And we - when we saw that Enceladus was so strange from Voyager suggested that something similar was happening on Enceladus. And the tidal heating was probably the source of the energy that was transforming the surface of Enceladus.

	And Enceladus is in a 2:1 resonance with Dione, a slightly larger moon further out. So every two orbits of Enceladus, Dione goes around one, so the rhythmic perturbation on Enceladus from Dione that increase the eccentricity of the orbit, so it's not quite circular, and the eccentricity is about 0.5%.

	It's actually quite similar to Io's eccentricity, and that's enough to distort Io as it goes around its orbit and its distance from Jupiter changes to produce the intense volcanic activity.

	So you can get a significant amount of heat into an object if it has this sort of eccentricity under the right circumstances.

	So there were a lot of papers published on the possibility of tidal heating in Enceladus. But there's an enormous variation in how much power that people thought you could get out of this tidal heating and anywhere from a few hundred megawatts, which actually isn't that much similar to what you would get from radioactive decay inside Enceladus to many gigawatts which could probably be enough to explain why Enceladus was so peculiar.

	And there were some limit on this place by the orbital evolution of Enceladus and how much energy Saturn could supply to the system to keep it warm and about 4 gigawatts but pretty uncertain.

	There was also an idea proposed just before the Cassini data started coming in that rather than Enceladus being heated by its orbit being (out of) around it, it would be heated by actually wobbling as - in a spin that it would not be - always keep the exactly the same (phase) to Saturn as most moons do but would actually wobble around Saturn for ways that might happen. And so the good shape will be distorted just by this spin wobble.

	So there were ideas kicking around back then that could produce a significant amount of heat.

	So moving on to Slide 4, there's always been a problem with these models.

	And that if you look at Saturn - or Enceladus as neither Mimas -- and here are pictures of Mimas and Enceladus pretty much to scale -- you can see Mimas is dead as a door nail, it's just covered in craters, nothing very exciting has happened to it in its history other than impact.

	But if you look at the numbers to what you would expect to tidal heating, Mimas is smaller than Enceladus, but it's much closer to Saturn or significantly closer to Saturn and its orbit is much more (out of) round. And so the tidal effects on Mimas should be a lot bigger than they are in Enceladus.

	And if you'd assume that they're the same in size with each other, you'd have ten times as much heat coming out Mimas as you do out of Enceladus; and this is clearly not happening.

	And so the probable explanation for this is that Mimas has a very cold rigid interior so although there are quite strong tidal forces acting on Mimas it doesn't really respond. It doesn't distort or it distorts in a very elastic way that doesn't put any energy into it because - and that wouldn't happen if it was cold.

	And - but Enceladus, because it's warm inside, it can - it's squishy, it can distort and respond to these tidal forces from Saturn, and that generates enough heat to keep it warm.

	So it's kind of a - it's a bistable situation that if you were to take Enceladus out of its orbit and put it one side like it cool down until it was hard and rigid, and then put it back in its orbit, it would never warm up again because it has to - it's a self-sustaining thing that the heat you're putting into it is what allows the heat to continue to be put into it.

	So that means you need some way to get Enceladus started in the first place. And there's been a lot of speculation about how that might happen.

Amanda Hendrix:	So are you suggesting that, say, if there was something to have gotten Mimas started way back when, then it could be in the same situation?

John Spencer:	Probably yes.

Amanda Hendrix:	Uh-huh.

John Spencer:	I mean there are other factors that come into it like can you maintain those eccentricities once you've got - once you got anticipation inside the moon that could make the orbit circular. And so, you need some other force like the influence of one of the other satellite, so combined with the influence of Saturn as well to keep the orbit eccentric so you can keep pulling energy out of the system without it just settling down into a circular (orbit). So there's more going on than that.

	But what will probably happen is if you heat Mimas up, it would have tidal energy for awhile, but then the orbit would just become circular because there aren't any forces maintaining its eccentricity.

Man:	Well, does it have as much water as Enceladus?

John Spencer:	Yeah. Well, it actually has more. You look on that little table. The density of Mimas is 1.2 grams per cubic centimeter. So in fact, Mimas is mostly water and Enceladus is significantly more rock, which maybe part of the story as well. It maybe that that rock allowed it to heat up more than Mimas in the first place.

	Okay. Let's go to Slide 5.

	The other part of the puzzle that had people's interest some several decades is the E ring. This is a diffused, faint - extremely faint outer ring of Saturn beyond the main rings. And it was first discovered in 1966. The image taken here I think was taken in 1981.

	And you see the ring, this little smudge poking out beyond the very exposed areas, the main rings on the left side of Saturn. And the ring seems to be dense at the orbits of Enceladus. And it's a blue color which means it's made of very fine particles, and those fine particles should not last very long.

	So there should - there has to be something that's continually producing fine particles to make the E ring and maintain it, and it seems to be concentrated at the orbit of Enceladus. So that immediately suggested that Enceladus was giving off particles in some mysterious ways.

	And even back in the early '80s, there were suggestions that there might be geysers on Enceladus that could be spraying out these particles. So it's not a new idea; I though it was crazy at that time because there are craters on Enceladus. And has to be clearly very old in order to have to have those craters and to have some regions to be active right this moment and other regions to be - have been dead for millions or billions of years seemed peculiar to me.

	So I never believed this. Just to go on record.

	But back in 1981, there was this illustration in National Geographic on Slide 6 showing geysers on Enceladus. And you can see the E ring in the background made up of particles from this. So National Geographic had it right substantially way back then. But it was all just speculation of course at that time.

	So moving on to Slide 7, there's more interesting stuff that was discovered before Cassini got there. One is that there is a torus of ring not just of dust but of OH gas around Saturn concentrating near the orbit of Enceladus.

	And again, producing this much OH - OH is a byproduct of water, if you, you know, destroy water molecules you produce OH. But there was just too much to be produced by normal processes of micrometeorite or sputtering with charged particles hitting the moon and knocking off OH molecule. And so this also suggested there was some sort of gas in the system as well beyond what you would expect.

	So moving to Slide 8, getting into the Cassini observations.

	As Cassini was approaching Saturn, it was scanning the system, this is in early 2004 with the ultraviolet instrument, and found there was not just the OH in the system but there was a lot of just oxygen atoms kicking around neutral oxygen atoms not (ionide) forming a ring around Saturn, again, sort of concentrating near the orbit of Enceladus.

	And so this was really strongly pointing to some source of water-gas or something that could break down and produce oxygen and OH.

	And then once Cassini got into orbit around Saturn, it started taking pictures of the system. There are actually a series of images taken in January and early February 2005 to look for plume activity on Enceladus because of all these works that are being done before.

	And in fact, there was a plume seen and this is an example of one of the pictures in the lower right, showing this sort of glow, this blob, the streak off the south side of Enceladus. But nobody really understood what was going on at the time and there were questions about the - could this be just some sort of lens glare from the camera. And so not too much was done with that at that time.

	But - then we had really our first close-up look at Enceladus in February and March 2005 when really got to see what it was like at much better resolution than Voyager.

	Then moving on to Slide 9, is one of my favorite pictures in those early flybys. They're very gorgeous, spectacular pictures taken from those first close-up looks, showing the surface even more intensely deformed than Galileo - sorry, than Voyager saw back in the '80s.

	This area here shows almost no impact craters and that means the surface is pretty young. Some processes have been destroying the craters that inevitably formed on the surface.

	You look towards the horizon and you see a very rugged place and mountains, clearly something very intense is happening on Enceladus here.

	Moving on to the next picture, this is looking at some of the older terrain and again, you'll see in these craters, the circular features that are clearly produced by impacts but they look like they've melted down and they're being absorbed back into the surface.

	The surface is clearly very weak, must be being heated from underneath so the ice gets soft and flows like a glacier and the craters just can't sustain the usual ball-shaped shapes.

	And then in the lower right of this image you again get the region where the craters are being completely destroyed by some very intense activity from the interior.

	So moving on to the next slide which would be Number 11, Cassini is wonderful in that it has so many different kinds of instruments that they can really attack a problem from many different directions.

	And one of the ways we were investigating Enceladus early on was with stellar occultations with the ultraviolet instrument which Amanda who introduced me is involved in this instrument.

	And the idea here is you watch a star pass behind Enceladus and you measure the brightness of the starlight as it winks out behind the moon and appears on the other side. And if there's no atmosphere, you expect a (nice edge), very sudden drop in the starlight as the star goes behind and the same in reversal as the star comes out again, and then find out exactly what we saw in the first occultation in February 2005.

	So here's the plot of the brightness of the star versus time and full brightness and then (binding) the star disappears and then (binding) it reappears on the other side. And for a moon this size, no big surprise that you would get an effect like that but we were still left with this mystery of what was going on there.

	But then there was indication not from the cameras that something else was happening on Enceladus. And the other thing that Cassini carries is a magnetometer, just like - can you imagine flying pass Enceladus with a compass and you watch the needle as you go pass. And the compass needle basically goes crazy as you fly pass Enceladus.

	And this plot shows on the first two flybys the direction that the compass needle would point during the flybys. And you see a very strong perturbation to the magnetic field by Enceladus as the spacecraft (fly passed).

	And this requires something to be blocking Saturn's magnetic field or perturbing Saturn magnetic field around Enceladus and it requires that thing could be bigger than Enceladus itself which - because the perturbation is spread over quite a large area. And this was extremely puzzling at the time.

	And in fact going on to the slide, in April 2005, we had a workshop at JPL to discuss what we've learned about Enceladus and the moon so far. Michelle Doherty who is on the magnetometer team was presenting her results and saying, "Look, there's something very strange going out to Enceladus. We really need to look more closely."

	And the next flyby was scheduled for July 2005, just three months later but Michelle argued successfully that instead of flying pass about a thousand kilometers of altitude which was the original plan that we shouldn't come in much closer and she was able to negotiate a flyby to 175 kilometers.

	And some of us were a little nervous about this because we've seen these images of the (pond) plume at the South Pole and this flyby was going to take pretty close to the South Pole and we would have hate to discover a plume by running the spacecraft into it. But we went ahead with this, and we were very glad that we did as it turns out.

	So Michelle really deserves a lot of credit for making a lot of these discoveries happened by being very quick on the mark - off the mark with - on interpreting the magnetometer data.

	Now moving on Slide 14, these are now the views from the third flyby where we approached over the South Pole. And really, this is the flyby that cracked the problem open and really told us what was going on.

	Earlier flyby (unintelligible) or earlier images that show some peculiar stuff down by the South Pole, some dark streaks. And these are visible in this (unintelligible) image on Slide 14 in the lower part of the image. They show up kind of bluish colored here. These are called Tiger Stripes early on and the name seems to have stuck.

	If we're going to the next slide, Slide 15, you can see a close up of this South Polar Region. And there is a series of four parallel bluish colored furrows on the surface, these Tiger Stripes.

	And then surrounding this, you can kind of see a boundary that's kind of scalloped going all the way round in the South Polar Region, the South Pole is maybe a third of the way of the picture from the bottom. And so something very different is happening at the South Pole than is happening elsewhere on Enceladus.

	Zooming in even further on the next slide, it's about the most detailed view we have so far of these four Tiger Stripe fractures.

	The blue color means that we have large ice grains here. It's like looking at the blue color of ice in a glacier. If light passes through enough ice, ice is pretty transparent, of course. But if light passes through a great thickness of ice, it will pick up a blue color. And so this means it's pretty (course-grained) ice here to pick this color here. So something is unusual happening along these fractures, these Tiger Stripes.

	Going on to the next slide, Slide 17, is a geological map of the South Polar Region. And this shows how astonishingly symmetrical the geology is about the pole. The red line in this plot shows the boundary - that scalloped boundary of the South Polar terrain. And you would see it's really perfectly centered on the South Pole, this very young region. And then the four Tiger Stripes in the middle which are labeled C on the plot are again perfectly centered right to the South Pole.

	And then radiating northward up towards the equator what look like tension fractures which again are pretty symmetrical. We don't really know what's happening in the upper left of it - sorry, the upper right of this diagram just because we don't have very good coverage there yet but probably very similar things happening there as well.

	And then this whole South Polar Region has little or no impact created at all. As I said, some other areas have quite a lot of craters. And you can see an area on the right hand - I'm one of these people can't tell left from right. You have to forgive me.

	But on the left-hand side is an area with quite a lot of craters, but there are almost none, maybe none at all in the South Pole. And that means it has to be really young if we have any understanding of the rate at which impacts hit or impact has hit Enceladus; probably less than a million years old.

	So moving on to the next slide, you can see a close-up now of the edge of the South Polar terrain. And what seems to be happening here is that the stuff from the lower right is pushing upwards and it's driving up into these fractures and it's splitting them open and it's just like an ax splitting open wood.

	You can really see just looking at the surface here what some of these forces going on. Ice is being forced up from the lower right into these wedges where it just (tries to) open to surface and causes these very fresh fractures that head northwards.

	So the images were pretty spectacular. The Cassini can do a lot more than take pictures.

	And the next slide shows the new infrared spectrum of Enceladus seen by the VIMS instrument, this is the Visible and Infrared Mapping Spectrometer which measures the near infrared spectrum where there's a lot of distinct 
de-absorption bands that can tell you about the surface composition.

Amanda Hendrix:	John?

John Spencer:	Yes.

Amanda Hendrix:	Can I interrupt you to go back to Slide 18?

John Spencer:	Uh-huh.

Amanda Hendrix:	What is the thinking on - you're saying that there's a lot of compression going on and material being forced up, it's cracked, and at the lower right.

John Spencer:	Right.

Amanda Hendrix:	Then - but then the crack is perpendicular to - let's see, the big crack is perpendicular to other what looks like crack…

John Spencer:	Yeah.

Amanda Hendrix:	…all the row.

John Spencer:	There's a crack that - or a furrow shall we say that starting on the right-hand side of the image cuts across more or less to the middle and then turns down towards the lower left. And that's the edge - that's the furrow that marks the edge of the South Polar terrain all the way around the South Pole.

	And that doesn't seem to be a tension fracture because it's - who knows what's going on there. But it looks very different from these fractures that are running up towards the upper left in the image.

Amanda Hendrix:	Right.

John Spencer:	And so the tension fractures we think are the ones that go up towards the upper left and that originate in this Y-shaped or wedge-shaped regions, you can see almost two generations of them here.

Amanda Hendrix:	Are these kind of old areas that are maybe not - does that maybe signify that the activity used to go up that far or used to (deal with) that far but now it's…

John Spencer:	I think it's all part of the same system. I mean, who knows. But I mean there - Paul Helfenstein has suggested and I'm not sure if he believes it, but just tossing out the idea that there might actually be subduction going on here, and that you have the ice moving northwards away from the Tiger Stripe region. And it's reaching some barrier here and it's being pushed down underneath the ice to the north.

	And so that trench that runs across from the right-hand side to the center and then down towards the lower left could be like a mid-ocean - well, not a mid-ocean ridge, but an ocean trench on the Earth where you actually have the ocean for diving down underneath.

	That's pretty speculative and I'm not sure that the images really provide strong evidence for that. But the idea that it stuff is pushing in that direction, splitting open across to the north, I think, comes across pretty clearly in these pictures.

Amanda Hendrix:	Yeah.

John Spencer:	Yeah.

	Okay. So going back to the next slide and that one is labeled Tiger Stripe Spectrum.

	You see it's just so wonderful how Cassini can pick a part of this problem and just look in so much detail. So we actually have maps of the spectrum across the surface.

	And in the upper right, you can see a map of the infrared brightness in the surface and the dark color of the Tiger Stripes there. They're dark there because this wavelength is absorbing. It's absorbed by water ice. So there's a lot of water ice there, it's - coarse grains of the ice has a lot of chance to soak up the light. And so the ice is coarser grained, it's just what was seen by the blue color in the visible images.

	The ice is mostly crystalline. It's not glassy-amorphous ice which some people might have expected. You don't see any heat radiation coming out of these wavelengths which you would if you really have high temperatures here.

	And then the next slide, Slide 20, shows the spectrum in a bit more detail.

	If you look at the end of the red arrow, there's a couple of little (bleeps) in the spectrum that are pointed out. These are due to organic materials something that come as carbon and hydrogen. We don't know exactly what. But that is really concentrated among the Tiger Stripes.

	And the image at the bottom right shows the map of these stripes of bright material where those Tiger Stripes are. And this is where those bands are strong.

	So there is organic material in here as well as a lot of coarse-grained ice. And there's a lot of carbon dioxide in that region. There isn't any ammonia for us which some people might have expected.

	And you can also - there are some gases you could observe here as well as the surface. And in particular, you should be able to see carbon monoxide coming off and that's not seen.

	Now moving on to Slide 21, now this is the stuff that I have worked on with my colleague (John Purl) to Goddard Space Flight Center. We have been looking at the heat radiation from all the moons as Amanda said, but looking in particular to Enceladus.

	And the image in the upper right shows the temperature map of Enceladus that we obtained on approach during this July 2005 flyby. And you see if you compare it to the view in the lower right which is kind of what we expected, you see warm temperatures up near the equator, but we also see this much warmer region right down at the South Pole.

	And in the predicted model, there is nothing special that should be happening down there at all, so this is completely unexpected. There's just no way to heat the surface at the South Pole by sunlight enough to produce a signature like this.

	And moving on to Slide 22, we can map out where the warm material is. And so this like a weather map I suppose of the surface temperatures on Enceladus. And we see this warm material pretty much corresponds exactly to where the Tiger Stripes are.

	And going on to the next slide, Slide 23, this is just showing kind of getting some feel for how our data works. We have a field of view or several field views which we scan across the surface. And here, I'm just showing the field view where we pick up the highest temperatures.

	And you see there's a cluster of warm material in the upper part of this image and near the ends of some of the Tiger Stripes in the cooler region. And then the warmest region of all is centered quite close to the South Pole or towards the bottom of this diagram. But there's quite a lot of variation across this area. So we really have quite a lot of information as to how the heat is distributed.

	So moving on to Slide 24.

	You can actually - if account for the resolution of the instrument, you can match this pretty well, the pattern that we see - if you assume that you have a region along the Tiger Stripes that's say at 135 Kelvin which is pretty darn cold, but it's a lot warmer than the - about 70 degree Kelvin temperature that you would expect the South Pole of Enceladus to be up.

	And along each Tiger Stripe, you have a region that you can vary in width from say 120 to 600 meters. And looking at the cartoon in the lower right and see how I just adjusted that manually to produce some variation in brightness along the Tiger Stripes, you can match the pattern that we see really pretty well.

	So we've seen similar kinds of temperature in all the Tiger Stripes and temperatures of at least 135 Kelvin. She's very peculiar.

	When you actually look at the spectrum and you look at not just to where the heat is coming from but how the heat varies with wavelength, while the heat radiation varies with wavelength, you again find that you match this best instead of having the whole surface be at a warm average temperature like 81 Kelvin that doesn't fit the spectrum, you need a small area of the surface to be much warmer than that.

	So about 1% of the surface is about 135 Kelvin will match the spectrum pretty nicely. So again, about 600 meters widths of warm material running along all of the Tiger Stripes will do this.

Man:	Then you're seeing about 60-degree Kelvin temperature difference then?

John Spencer:	Something like that, yup.

Man:	Yeah. Because, you know, the problem isn't some of this material that's been released; it looks like there's only a 10-degree temperature…

((Crosstalk))

John Spencer:	Well that's the thing; it's a little hard to present. Because what we see, if you just look at the average temperature in this region, you see from the earlier images our resolution is not so high. We're blurring. We're mixing together warm material and cold material in every spectrum we take.

	And so the average temperature is only a few - maybe 85 Kelvin or something, you know, in our field of view. But when we look at the individual - when we look at the spectrum, we can actually see it's not just all material for that average temperature, it's a small fraction of the material with a much higher temperature.

Man:	Okay.

	It makes a lot more sense. Thank you.

Amanda Hendrix:	Then, is that small fraction indicated by the stars?

John Spencer:	In, let's see, in slide - in the previous slide, Slide 24…

Amanda Hendrix:	Yeah - oh I'm sorry. I was still on 24. I didn't know that we…

John Spencer:	Yeah.

Amanda Hendrix:	So...

John Spencer:	Yeah. The stars are just showing where the warmest region in each - on each Tiger Stripe is.

Amanda Hendrix:	Right. Okay.

John Spencer:	They're not actually coming - showing very much so probably we would leave them out but they're just indicating - highlighting that the temperature along these Tiger Stripes - well, the amount of heat along these Tiger Stripes (lot). And those are the warmest regions.

	Okay, so going up to Slide 26, we can look at the total heat coming out and make some assumptions. And we get about 6 gigawatts of heat coming out. And there's quite a lot of uncertainty and this depends on what you assume for the brightness of the warm materials for instance which we don't know directly.

	But this is a lot of heat. Remembering the models that we had before Voyager, had a hard time producing much more heat than this; a few gigawatts were about the most you could get out of the system from tidal heating. And so that's pretty interesting.

	Then Slide 27 shows, well, we have a few observations at higher resolutions where we scan the surface - not the whole surface, but we just got scattered looks across the South Polar Region.

	And the warmest spectrum of any that we saw has temperatures of about 145 Kelvin maybe 150. It's hard to say exactly what the highest temperature is but something in that ballpark and about 5% of that particular spectrum is of those temperatures. This is because we are zooming in closer and we're filling more of our field view with warm material along the Tiger Stripes.

	So moving to the next slide, this is - we do the press release pretty soon after this discovery where we actually can locate the warm material along the Tiger Stripes here. And we get the warmest material right over the stripes.

	So we don't have information over most of this image but we do have information along the strip, and the warmest stuff is right over Tiger Stripes. So heat is coming out of these things.

Man:	You see this is exactly the slide that I found confusing.

John Spencer:	Right. This is the average…

((Crosstalk))

John Spencer:	…temperature.

	And even in that middle square, the one at 91 degrees, you've got a mixture of cold material and warm material. And the warm material is much warmer than 91 degrees. That's simply the average temperature.

Man:	Okay. So what you were saying is actually 130 to 140 Kelvin or something like that.

John Spencer:	Right.

Man:	Okay.

John Spencer:	Trying to put all that on a nice press release graphic would have gotten way too complicated.

Man:	Yeah. But you know what, the way the press release came out, it looked like there was only a 10-degree difference. And scientifically, that just didn't make sense to me.

John Spencer:	Yeah. Yeah.

	The other thing is that - I can't remember the exact time, but we may not have figured out that we need much higher temperatures when this came out. This is only about a week after we discovered this.

Man:	Right. Okay. Thank you.

John Spencer:	We do all this.

Man:	Uh-huh.

John Spencer:	Okay. We also got a look at the North Pole because we flew - we approached Enceladus from the South Pole, but then right after we turned back and look to the North Pole.

	And here we get a totally different story in fact the highest temperatures, all the temperatures we see at the South Polar around 30 degrees Kelvin; incredibly cold. And that's because the South Pole has been in darkness for many years since I think about 1997. There's been no sunlight forming on it so it's had a long time to cool off.

	So you'd expect the temperatures to be this cold. But it means you don't have a lot of heat coming out from the interior. And the North Pole keeping it warm, is just been - was heated by the Sun in the early '90s and it's been cooling off ever since.

	And the Voyager images of the North Pole that I showed here again show pretty heavily crated region. And so, you wouldn't expect necessarily to see a lot of heat coming out of there. So that's pretty consistent that the North and South poles are very, very different.

	Moving on to Slide 30, this is a graphic where I'm going through all - put together the evidence we have from the different instruments and see how it all hangs together.

	So here we have - this is like a slice through Enceladus, the disc) of Enceladus centered on the South Pole and it's just showing where the things are that we're seeing that are interesting.

	And so around the South Pole, we have this warm material that extends about 10 or 15 degrees away from the pole and that will make more sense in a minute.

	Next, I'm going to go back to the ultraviolet instrument which earlier had seen nothing in its stellar occultations by Enceladus. But in this case, we were able to arrange for occult - for a star to pass behind Enceladus under the South Pole.

	And in this case, we've got a very different story. The plus on the right shows that the brightness of the star decreases gradually before it hits the edge of Enceladus and then drops. But when it comes out again, it's on a nice edge again.

	So there is a - hang on a moment. I've got…

((Crosstalk))

Amanda Hendrix:	…everybody.

John Spencer:	Just a moment.

Amanda Hendrix:	All right, John.

Man:	(Ivan calling).

John Spencer:	Okay. I'm back. Sorry about that.

	And so we're seeing gas over the South Pole and the ultraviolet as well confirming what the magnetometer saw. And not only do we see those gases, we can tell what kind of gas it is from the spectrum. And it seems like mostly water vapor.

	And we can measure the amount of water vapor from the rate of which it forms off with height. We can get some idea of the speed of the water vapor as it is escaping.

	It's pretty fast. It's about 400 meters per second. And the escape speed from Enceladus is only about 240 meters per second. And so the gas that's coming out is just leaving Enceladus completely.

	And when you do the numbers, if this has been going on for the entire history of Enceladus, you'd have lost about 20% of Enceladus' mass in that period of time.

	So this is pretty major phenomenon that's going on. It doesn't mean that Enceladus has lost 20% of its mass because who knows how long this has been going on. But it could be that high or it could even be higher.

	And this amount of water vapor coming out of the South Pole is just about the right amount to produce the OH and the H clouds that have been seen previously. So it's making a nice story here.

Man:	How long was that 20% loss - over what period of time…

((Crosstalk))

John Spencer:	I believe over 4 billion years.

Man:	Four billion?

John Spencer:	Billion.

Man:	Billion.

John Spencer:	Yes, so over the edge of the Solar System basically.

Man:	At some point, it started running out.

John Spencer:	If this had been happening continuously.

Man:	Because you just released a picture I guess taken last August that looks like a gigantic plume of material coming out.

John Spencer:	Yes, there is a gigantic plume. Yeah.

Man:	That's exactly what I was wondering.

John Spencer:	Yeah. And I'll show some of the images shortly.

	And I'm kind of showing things in chronological order up here because as we were doing these discoveries in the summer of 2005 all we had were those early images which didn't show the plume very clearly and we weren't sure if it was real or not in the images. So we were inferring most of this from these other instruments.

	So the next slide, Slide 33, now adds the next element to this picture of what's going on.

	We have a series of circles under the South Pole of Enceladus that tracks - the bigger the circle, the more water vapor was seen by Cassini at that point above the South Pole. And then the extra show where there was no water vapor seen at all in previous occultations.

	So again, we see this cloud of water vapor right over where the warm material is at the South Pole.

	So going on Slide 34, even better we actually flew through the edge of this cloud of water vapor and we have a mass spectrometer on board Cassini that can measure its compositions directly until we actually were able to - this material have been spewed out of the moon just minutes earlier, we could bring in to our instruments and do perform a chemical analysis on it.

	And consistent, we've got the ultraviolet instrument so it's mostly water vapor. But there's also carbon dioxide, nitrogen, methane, acetylene - what's the other one there?

Man:	Propane.

John Spencer:	Sorry?

Man:	Propane.

John Spencer:	Okay. I'm forgetting my Organic Chemistry.

	But yeah, just from what we could on this one quick pass, there's quite a rich brew of materials here. And so the pie chart kind of shows how it's mostly water vapor but with a significant amount of these other stuff.

	You know, this stuff would not pass all the quality standards. It's very contaminated by a lot of other stuff.

Man:	So what would you estimate, if you can, the composition of the organics? Is this about 1%?

John Spencer:	Yeah. And these are the only molecules that are being identified with any certainties. Very simple organics: methane; acetylene; and propane. But there's probably a lot more at the less than 1% level that the instrument just was not sensitive enough to see. This is all that we know directly so far.

Man:	Thank you.

Man:	Basic elements of life.

John Spencer:	Well yeah. Right.

	And it was pointed out pretty early on by (Jeff Kargel) that if you have water vapor occurring - water-ice when it condenses can incorporate lots of other molecules into it. They get trapped in the molecular structure of the water-ice; that's called a clathrate.

	And you can get, you know, about 10% of these other simple molecules trapped in water-ice as a clathrate. So that means this could have been the source of the material that there was actually a clathrate composition in theory that inside there's been - it's decomposing to produce this.

	But it's pretty interesting there's no ammonia here. Ammonia has been a favorite anti-freeze for this Saturn system and for other regions of other moons in the outer Solar System for a long time.

	If you add ammonia to water, it should lower the melting point by about 100 degrees so you can produce volcanic activity. You can alter the surface at much low temperatures and so it's been very popular concept that these objects have quite a lot of ammonia in them and that makes it easier to have volcanic activity and explain some other features we see that the reason is ammonia, that's the one thing we don't see.

((Crosstalk))

John Spencer:	Now, I've plotted on the same diagram the trajectory that the spacecraft took pass Enceladus. And now the circle showed the density of gas it was measured by the spacecraft in the region that it was flying through.

	So you see a gradual increase in density as we got closer to the South Pole of Enceladus and then a pretty rapid fall off as we went pass up towards the north.

	So again, this hangs together, there's a big source of gas at the South Pole.

	Slide 36 now shows dust observations. We can also measure the dust particles themselves as they hit the spacecraft. There's an instrument that will count those particles, estimate the sizes, and it can actually estimate the compositions as well. And mostly saw a particle of bigger than a few microns in this observation.

	And again, you see the green circles represent how much dust there was and you see there was a lot of dust (unapproached) and very little dust on departure up to what's enough.

	So again, we're seeing this all - gas over the South Pole. And you can make it a tricky calculation but you can make some estimates as to how much dust has to be coming out of the South Pole. And it's at least a few hundred grams per second and that is getting up to be enough to produce E ring. So we really think we've found the source of the E ring here.

	And then the magnetometer of course is taking data as well and if you're looking at the PDF you won't probably see the nice transition here but on the PowerPoint I first showed the dates from the first few flybys and then the dates from the third flyby which shows again a big perturbation. This is the day of Year 1 '95, a big perturbation just to the south of Enceladus and so again consistent with everything else we've seen.

	And then, the next slide shows the famous image of the plume that was taken quite a few months later. This was in November 2005. Cassini got the chance to look back at the night side of Enceladus and see the surface backlight by the Sun which is a good way of seeing fine particles like if you want to look at smoke, it's a lot easier to see smoke looking towards the Sun than it is away from the Sun and these very fine particles show it best that way.

	So here's a few now that flinches the reality of the plume that was seen earlier by Cassini earlier in the year, and since all these very fine jets coming out from the South Polar Region which on these images to the lower left.

	So fine jets visible on the left-hand image and then on the right-hand image is enhanced version of that but shows the fine jets suggest part of an enormous broader plume of material coming out here.

	And the next slide, 39, just says, "There are some peculiar things about this." The particles fall off very rapidly, the brightness fall off very rapidly within tens of kilometers at the surface. And that means that most of the particles are not actually escaping even though most of the gases escaping the particles and mostly not going that fast and then falling back to the surface.

	But then above the 50 kilometers or so, then there's a much more gradual fall off and that means that a significant fraction of the particles that maybe only about 1% of the sum are actually escaping.

	So from the image, we can tell a great deal about what's really happening close to the surface that we could conquer with the other instrument.

	And Slide 40 then puts the plume image in the context of all these other observations. And so now, we can see the plume again exactly corresponding to where the warm material is on the surface and the dotted line kind of extract lights out where the edge of the plume is from that one image which doesn't cover a very large area.

	And you see that the spacecraft just kind of scattered the edge of this plume as it came passed. So the densities of gas and dust that we've measured are much smaller that we'll potential be able to measure if we get closer to Enceladus.

	Slide 41 now shows you more wonderful data from that same observation by the VIMS instrument which can measure the infrastructure red spectrum of the plume material itself. We can actually measure the spectrum of those particles as they're being injected. And I won't go into too much detail here except that the green spectrum in this (flap) in the lower right shows a strong signature of water ice.

	So again, we're conforming that the particles coming out are mostly made of water ice but it is very fine grained water. That's the only way to match the spectrum.

	Slide 42 just talks a little bit about the heat, the power that we're seeing coming out. We see about 6 gigawatts of heat being radiated. But if you'll actually look at just the energy required to convert all the ice in the interior to water vapor, that's another gigawatt right there just to supply that rate of water vapor.

	And so, 7 gigawatts at least coming out of Enceladus and this amount of power is kind of hard to do by title heating if we want to maintain this long term. And in fact, there's a paper just coming out now that it says it's completely impossible to maintain it long term by (unintelligible) and wisdom.

	And that's kind of awkward because if it's not maintained long term and it's only happening occasionally, it's a little interesting that we showed up at just the right moment when it happens this very (unintelligible), when it happens to be occurring. So, you know, maybe that's really what's happening but it makes it a little unlikely. So we're not really sure to make of that yet.

	And then as we said earlier, clearly we have title heating going on. We sort of can't think of any other ways in title heating to produce these many gigawatts of power.

	And so, we still need to wait to get Enceladus started but not Mimas. And it has been suggested that maybe when Enceladus was formed, it formed very, very early shortly after the Solar System condensed out of materials from a supernova. And that supernova, 4.5 billion years ago would have has a lot of - produced a lot of very intense radioactive elements that would have produced enormous amount of heat and aluminum 26 is the most popular of these.

	So if you had that much heat around early on, you could certainly have warmed up Enceladus and kick started the process in line of the title heating to continue since then. But there are other problems like if there was much aluminum 26 around when Saturn formed, it would have melted all the moons and that's not necessarily what we see.

	So it's possible but we hope that there's some other explanation I guess. It's possible that just regular radioactive heat, the kind of keeping the Earth warm right now would be enough to kick start the title heating but you kind to have to push the models to make that work. Or maybe there has been an impact on Enceladus that knocked it out of its circular orbit and produced a temporary pulse of heat that then allowed the title heating to get going. We really don't know what's going on there.

	And if you look at the total heat just coming of the South Polar Region where it's concentrated, the amount of heat flow if you just have ice that is the heat is being connected through that, that light ice has to get pretty warm pretty near the surface and it would be molten at the depths of only 20 meters, 60 feet below the surface if that heat was just coming out by conduction through the ice. So we've got liquid water very close to the surface if that's really what's going on.

	I'm going on to Slide 43. This is an idea that has caused a lot of discussion. The paper on the imaging results pointed out that the amount of gas coming out of the surface seems to be quite similar to the amount of ice coming out of the surface if you make a lot of assumptions about what the processes are. And it's kind of hard to produce that much ice from condensation of gas.

	And so there, the argument made in that paper is that, the best way to produce that many ice particles with that amount of gas would be to have boiling water near the surface. So they're suggesting the water really is - liquid water is really occurring very close to the surface, it's boiling off into a vacuum and that produces all the ice particles we see.

	And this is - it's a plausible argument. One, complicating factors if you actually look at the relative speeds of the ice - of the dust and the gas that the amount of - the rate in which you have to produce ice particles is a lot lower than the rate in which you have to produce gas particles. And maybe you could do that by condensating - condensing vapor but if you just look at this straight then, yeah, yeah, there's an arguments maybe liquid water close to the surface though.

	On Slide 44, it talks about a recent paper by (Supiffa) and others that I was involved in. Again, looking at the abundance of these non-water constituents, the carbon dioxide and the nitrogen and the organics, that a similar thought you'd get from clathrates and if you expose clathrates to a vacuum, you might be able to get an explosive reaction where they decompose rapidly and produce these jets of vapor which could explain what you see and maybe an explanation for the plumes. But you need pretty high temperatures even say probably over 190 Kelvin in order to get the water to vaporize that we see in the plume.

	Another puzzle that is addressed on Slide 45 is why would this be happening at the South Pole? Remember we have this amazing symmetry of the geology around the South Pole. Until the idea that (Francis Nemo) and (Bob Papolado) published last year is that, suppose you just have in some random place on Enceladus you have heat concentrated by title heating processes, however you want to do that near the surface or even deeper in the interior.

	You could heat up the surface and you produce low density material there. And so, you have a ball of ice that's mostly symmetrical, most of the same everywhere but there's one region that's lower density doesn't weigh as much.

	That produces - if it's spinning, that produces an unstable situation. And in fact, the way to make it stable again is for the body to rotate so that the low density material is concentrated at the pole. That then becomes stable and it will spin steadily after that.

	And so, it's possible that the activity did not start at the South Pole. It started in some random place and that causes the moon to flip so that the activity was then centered at the South Pole.

Amanda Hendrix:	But then you have to have a mechanism for starting the activity in some random in place?

John Spencer:	Yeah. I mean that's where the (arm waving) comes in.

Amanda Hendrix:	Right.

John Spencer:	And this model doesn't address that in any detail. It simply explains why you would have this going into the activity with the pole.

Amanda Hendrix:	Do they suggest particular ways that you might get that there? I mean would that be like an impact or something?

John Spencer:	No, that's thinking of internal process. I think some kind of (unintelligible).

	There are processes - once you get a zone of weakness, you get more deformation there and that produces more heat there which could make it weaker and so you could get a ran away effect where a small area of weakness because of the title heating becomes a much larger area of weakness. And so, it's plausible that this would actually work this way.

	But if this is really true and we have a low density region below the South Pole, you might be able to detect the gravity anomaly from that. And so that might be something that we could see in future flybys.

	Moving on to Slide 46. There have been other works recently on the heat production mechanism. And one way to produce these amount of heat is just simply, you know, you have a fracture and the (tights) are pulling on Enceladus and that's causing motion along this fracture every day is Enceladus. (Orbit Saturn is different from Saturn). And you can calculate from what - that fracture that the (unintelligible) should be and how much heat should be produced.

	And you can produce models like this diagram here that shows how the temperatures are warmer along a fracture below the surface. And you can produce quite a lot of water vapor this way.

	And if you assume that most of that recondenses near the surface, it freezes out until it gets to the right (unintelligible) surface region but then warms those regions up in the process. But some of it escaped to produce the plumes. You can actually make the numbers work out pretty well from the amount of heat we see coming out and the amount of vapor we see coming out.

	But to get the amount of stress required to make the numbers work, you actually need to have an ocean inside Enceladus so that the ice layer on top is free to distort to the full extent as Enceladus orbits Saturn.

	So I won't call this proof of an ocean but it's kind of pointing to the direction that like Europa around Jupiter and maybe an ocean beneath the surface. Even if there isn't really good water right at the surface that the imaging team suggested that there might be an ocean lower down because this model doesn't require liquid water very close to the surface.

Amanda Hendrix:	John, how is an ocean consistent or not with magnetometer measurements compared with say, Callisto?

John Spencer:	Yeah, I think magnetometer data just aren't good enough yet.

Amanda Hendrix:	Okay.

John Spencer:	And they may not get good enough on the Cassini mission because this enormous signature from the water vapor cloud which you don't have to deal with a Callisto or…

Amanda Hendrix:	Right.

John Spencer:	And that confuses the issues quite a lot because I understand it.

	Okay, so Slide 47 now shows the - in this model you can actually predict how the heat should vary along the Tiger Stripes and this one of those predictions from this paper. And you can actually compare this to models. You can look at what the signature should be. The heat signature should be at the resolution of the Cassini data that we have so far.

	And, you know, kind of matches in some ways. We have a warm area in the upper left and warm area in the lower right but then we have kind of cold region over the pole in this model which doesn't match the data than the data round the left and the model predictions that are on the right.

	So it's, you know, it's promising but it's clearly not the whole story or it's not quite that simple.

	Okay. And moving on to Slide 49. We've had another look at the South Pole since then with the CIRS instrument to look at the temperatures again. And the lower right image shows the - it's actually a more direct view at the South Pole than we got in July 2005 and we see the hot spots still there, still looking pretty much the same, still about the same brightness.

	But an interesting thing is you can see from this diagram on Page 49 this image that, we were looking pretty obliquely in 2005. But in 2006 we were looking almost straight down on the South Pole.

	And so, we thought we might see a lot higher temperatures there if we're really seeing narrow cracks with hot material in them as we're looking straight down and we'll see deeper into the crack and we should see more heat. But we actually see pretty much exactly the same amount of heat.

	This is a spectrum from late 2006 and the temperature we're getting are 127 Kelvin. It's about the same as what we've get on the earlier data.

	So we're not seeing a lot of hot material in cracks. We're seeing - we think most of the heat we're seeing is from a warm surface away from the cracks and not from the cracks themselves.

	Power is about the same though it is somewhat uncertain on Slide 51.

	And Slide 52 shows the map again of the temperature distribution across the surface and given the uncertainties, it's pretty much the same. We see again the warm region in the upper right and - but the main warm region is in the lower left of this diagram. So things haven't changed too much in the course of a year and a half.

	Now Slide 53 shows another example the amazing days that the Cassini is getting. This is an image taken looking back towards the Sun during a solar eclipse last September. And here's Enceladus now in the heart of the E ring.

	And if you have heard any doubts about whether Enceladus was creating the E ring, this should dispel them; you see that it's right in the center of the ring. It's really messing up the ring as it plows through it.

	And you see jets stream is coming off. Enceladus itself is just a little black dot in the middle of this picture because we're looking at the night side we don't see much light from Enceladus itself. Then the very bright dot beneath the dark dot is the plume; very bright (lily top) by the back illumination of the Sun. So there's a lot of incredible stuff going on here.

	So Slide 54, I'm going to start talking a little bit about future observations. We have - the nominal mission of Cassini extends through the middle of 2008. So we have about 15 months to go. Under a plan for an extended mission after that and these are not official yet. We don't have official approval of those but we have been making plans because we need to, to get started on the planning. And we're certainly hoping for a long and productive extended mission but it's still tentative at this stage.

	But one thing we do know we're going to do is next March - oh actually, oh today. Today is actually a big day for Enceladus because we're getting another look at the plumes at high (phase) angle today.

	If you check the (raw image) Web site on the Cassini Web site and maybe by tomorrow I'm not sure but maybe some new detailed pictures of the plumes better than we've seen since that first view from November 2005. So we're looking forward to that view. But then the next time we get really close to Enceladus will be in March next year.

	And the current plan is - sounds crazy, it's a flyby with an altitude of 25 kilometers. But that's currently what we're planning to do the navigation. People think that's a safe altitude to fly.

	And then, just to - less than a minute later we fly over south to South Pole in a much higher altitude. And we will be then be able to turn back and look at the South Pole maybe some 10,000 kilometers range a few minutes later.

	And the only problem with looking back at the South Pole at this time is that - the - Enceladus goes into Saturn eclipse. The lights go out right as we go pass. And so we won't be able to get anymore detailed pictures of the South Pole which is a region - which is a shame. But we will be able to look at the temperature distribution. We'll be able to map the temperatures there and we're certainly looking forward to that opportunity.

	Slide 55 shows the trajectory of the diagonal white line shows the plan flyby in March in 2008. And you can see how incredibly close to the surface we get near the equator and then we go right through the plume, much closer than we did on the last flyby. But the time ticks every 10 seconds and that's how quickly this happens; very little time to gather this really high priority science in this period.

	Slide 56 then talks about the extended mission where we have now decided what trajectory we're going to fly if this is approved. And not surprisingly considering all we've discovered so far, Enceladus is a major priority for the extended mission.

	In fact there are seven additional close Enceladus flybys planned. And we're hoping to take pictures of the Tiger Stripes showing details of small or tens of meters or even a few meters should be really spectacular. We'll be looking at the plumes themselves and the favorable illumination conditions that tens of meters per pixel that I'm sure will be very spectacular too.

	We'll be measuring the heat radiation and the composition details of - the smallest is about a kilometer. The best data now is about six kilometers. I will be doing more searches for the atmosphere using stellar occultation, be looking at other places not just at the South Pole.

	And then now with the (unintelligible) instruments that are measuring the plumes as we fly past it, we can measure the composition of individual dust grains, we can measure the gas composition much more sensitively that those being done before. And we may find some more exotic and interesting…

Man:	What's the density of the surface of the moon?

John Spencer:	I'm sorry?

Man:	Density of the surface in general, I mean, the surface. If you walk at it, would you cave in or…

John Spencer:	Oh, we don't really know. The surface is very fluffy. But that's only telling you about the top few millimeters.

Man:	Okay.

	Could you land something on it, you think?

John Spencer:	We're looking into that. I think you probably could. But it would be very hard to be certain in advance that it would be successful. I think it's probably the surface is like freshly fallen snow because that's kind of what it is. The reason it's so bright is that there is dust falling out unto the surface from the plume and from the E ring all the time.

	And so, yeah, it's - I suspect it's kind of a soft surface but - that you could land something on it.

Man:	Thank you.

John Spencer:	And then, we look at the plasma and the gravity field as well.

	Slide 57 gives a table of the planned encounters. The top one is the one that is in the main mission and we know is going to happen on 12 of March, 2008 but then we have these other seven with different geometries, and different distances, and different speeds as we go by. And so, we're now in the process of deciding what we do on all these encounters.

Man:	Absolutely fascinating.

John Spencer:	So this is Slide 57 and going unto Slide 58. This now shows on the right-hand side how old these additional trajectory fly through the plume or not. So the bright green line is a flyby on August 11 and the sort of all this (drab) I suppose diagonal line now is the flyby to (unintelligible) next March.

	But you see the red line shows two of the flybys that we're planning in the extended mission which go even deeper in to the plume. And we'll show a lot of the sample of the plume with even more - you know, look at even denser regions and get more accurate measurement of the composition.

	And then the orange line coming across from east to west are the closest flybys that we're currently planning on orbit; it's 120 and 130 which are just a 100 kilometers over the plume source. And we have a working group that Amanda is actually leading to try and figure out if this is crazy or not, or whether it's really safe to fly that close over the plume.

	If we run in to a particle that's bigger than a millimeter or two, it could destroy the spacecraft. And so, we're trying to convince ourselves that there are no particles that big that could get that high. But it's not an easy calculation to make and we're being very cautious about that understandably.

	And the image in the lower left shows the kind of pictures we could take. Here again is a crescent image where we are hoping we could zoom in on the South Pole. This white square is the field to view of our narrowing angle camera on the spacecraft. We can take - we can fill an image and fill that square with a high resolution image and get incredible detail on these jets of dust as they come out from the Tiger Stripes. So that's going to be something to look forward to.

	The next slide shows the kind of imaging of the surface we hope to be able to get. This is a view of the surface during the first or one of the first flybys where we'll be able to look at the surface showing an image - again, the white square shows the size - the field view of our camera as we pull away the field view gets bigger. But we'll get some image zooming in on these very small areas you see on the left-hand side and then progressively larger areas.

	The red vertical rectangle shows the field view of our - of the CIRS instrument, the temperature mapping instrument so we can measure temperatures along stripes like that. But each of these positions are - or wherever we want to put these views on the surface as we fly pass it.

	And the little inset in the lower right shows what the surface of Enceladus looks like at this very high resolution. This is an image that was taken - are only a very high resolution. We have - and you see a very rough surface apparently covered in boulders which is not at all that we expected. That is the kind of information we can get over much more and much larger areas during these close flybys.

	So the last slide, Number 60, summarizes all this. We found kind of a holy grail of Planetary Science really which is a living ice world. Most of the bodies in the solar system, most of the solid bodies, we see traces of geological activity from the past but we don't see much going on now.

	The (rest) of course is very geologically active and Jupiter's moon, Io, has it's amazing volcanoes. But this is the first ice world we have seen where geology is happening right under our noses. And it allows us to study processes that we think have happened in other places in the outer Solar System, another ice world but there we could only pick over the remains of what happened and try and reconstruct it; here, we can see it happening in front of our eyes.

	And we're pretty sure that the heat source is (tight) and we can't think of anything else that would produce this amount of heat but we sure don't understand how this much heat is coming out.

	And there are good arguments that can be made that there's liquid water close to the surface. But then, there are - you can think of ways of explaining the - what we see without liquid water near the surface.

	So we don't know if there is liquid water near the surface but it's certainly plausible under stronger arguments that there is heat deeper in the interior. Also, there's liquid water deeper in the interior.

	So we have probable liquid water somewhere inside there. We have organic molecules, it is an area where - a region where we can imagine that life might gain a foothold and it maybe - arguably the most promising place in the Solar System that we currently know if where life could exist right now. There's a big ocean we're pretty sure under the surface of Europa, the big moon of Jupiter, a much bigger ocean possibly inside in tiny Enceladus.

	But that ocean is locked under kilometers of ice. We have no way of directly sampling it but we can conceive of technologically right now whereas to sample of the interior of Enceladus, all you need to do is fly through the plume and we're already doing that with Cassini.

	So we have an opportunity for really understanding what's happening here maybe clinching whether or not there is liquid water here. And with all these extended mission flybys that we're planning on Cassini, there's a chance of very dramatic discovery still to come and we haven't seen nothing yet.

	And that's basically my talk and I'm happy to entertain more questions.

Amanda Hendrix:	Great. Thanks, John.

	Does anybody have some more questions?

(Ken):	Yes, I do.  This is (Ken) of North Carolina.

	Two questions. First off, my nuclear science is a bit rusty. What are the decay products that aluminum 26 and is there way to sample for those as you're flying to the plume?

John Spencer:	Aluminum 26 decays in (unintelligible) 26. And we know aluminum 26 was present in the (Earth) because we see excesses of magnesium 26 in meteorites. And so we know it was present when meteorites formed.

	I think it's a small fraction of the interior. I don't think it's anything we could detect - or we could detect magnesium 26 but we wouldn't know where it came from.

	It's all in the details of not just other magnesium 26 atoms but when where they incorporated into the object, where they incorporated as aluminum or as magnesium. And to do that, we need to do a detail chemical analysis that we couldn't possibly do with Cassini; maybe on some future missions.

(David):	John, I have a question. This is (David).

	Can you hear me?

John Spencer:	Yes, loud and clear.

(David):	Okay, thanks.

	It would be fascinating to hear a comparison of this world versus Callisto and Europa.

John Spencer:	Yeah.

	I mentioned a little about that in some of the - has a lot in common with Europa not so much with Callisto. (Unintelligible) geologically dead; there's evidence for a liquid water in the interior but very little happening on the surface.

	But Europa and Enceladus have a lot in common. Europa is much, much bigger. It's a bout 10 times the diameter of Enceladus.

	And we have a lot more information about the surface geology of Europa, and that's pretty much convinced most people that there is a layer of ice, we're not sure how thick that layer of ice is, but it is overlying a liquid water ocean. And like Enceladus, it's tightly hidden. The surface has some features that look a lot like those in Enceladus. We see fractures that look sort of like the tiger stripes actually, and a lot of other weird stuff as well.

	The big difference is that on Europa we see no sign of activity right now. It's clearly been active fairly recently in geological terms in the last tens of millions of years. But we see nothing happening right now.

	And that has tow implications. One is that it seems to be a less energetic system. The other is that, it's just a lot harder to study. We see - again, we're picking over the remains of things that happened a long time ago trying to understand them rather than being able to watch them right now.

	And so, yeah, there's probably an ocean inside Europa but we have no way of sampling it, as I mentioned. It's a plausible habitat for life but it's going to be very, very hard on any mission that we could currently build to determine whether there is life down there whether learn more about the conditions in the interior.

	Whereas, in Enceladus that's much easier because it's letting it all hang up.

(David):	So it's like the shift from has there been light in the solar system to, you know, there possibly is.

John Spencer:	Yeah, I mean who knows. We know so little about how - what it requires to sustain life, having only one example, and so lots of why we care so passionately about looking for it elsewhere in the solar system. But it also means it's really a (unintelligible) that the, you know, the area - if there is liquid water in Enceladus which there probably is, it's a pretty small area, and like compared to the volume of the earth's oceans or Enceladus' oceans, it's a tiny volume. Maybe that just isn't sufficient to sustain life over the age of the solar system even if got started.

	Who knows? We can't really estimate the probability of life on Enceladus. It's just - it's a good place to look because there is probably liquid there and that's, as far as we understand, is one of the prerequisites for life, at least any kind of life that we can imagine.

(David):	Uh-huh.

John Spencer:	Liquid water is very rare in the solar system and here's a place where it probably exists and probably exists very close to the surface where we can study it.

	So, yeah, if I had to bet money, I would bet there isn't currently life on Enceladus, but I sure would like to look closer and find out.

(David):	Uh-huh.

	Okay, thanks.

Man:	Hi, this is (unintelligible).

	I was just curious. Your New Horizons measurements at Europa, they add anything to this question?

John Spencer:	They add a little bit. We're having a press conference next week and we'll talk a little bit about Europa there. But we did not come very close to Europa at all with New Horizons.

Man:	Right, right.

John Spencer:	I've seen nice pictures of it, but they're not nearly as detailed as the pictures that Galileo got as it - when it was orbiting Jupiter in the 1990's.

	So…

Man:	But didn't…

John Spencer:	…we can look at the problem from a couple of slightly different angles. We can look at some type of the surface under different lighting conditions than Galileo and see new features. We have quite a lot of ultraviolet observations of Europa that will tell us more about its atmosphere. And they may in turn tell us more about what's happening on the surface and in the interior.

	But it's going to be small advances. I doubt that we're going to break open the subject of Europa and the possibility of life there with New Horizons. But we'll add some interesting new data.

Man:	Thank you.

John Spencer:	But, yeah, check into NASA TV next Tuesday I think at exactly what time, but we'll be saying a little bit about what we've learned there.

Man:	Right. I saw (unintelligible) because of the different lighting conditions there was something more you were going to learn.

John Spencer:	Right. We'll be looking at very oblique illumination regions of the surface that weren't seen under those conditions by Galileo. We did that. And we do - we do see some stuff, but it's, you know, it's subtle new stuff. Don't get too excited about that particular thing.

Man:	Okay.

	And one other question too. What is the margin of error for these close flybys?

John Spencer:	Well, yeah. Clearly, the navigation people thought long and hard about this before recommending 25 kilometer flybys. And they think their accuracy is 30 kilometers I think.

Man:	(Unintelligible).

John Spencer:	It's amazing what they can do; it just astonishes me.

	There - so the concern with flying that close is more about: well, what if something went wrong with the spacecraft and the (unintelligible) came on when we weren't intending them to and they knocked it a little bit of course? Is there any conceivable way where you could have multiple failures that would cause you to collide with the moon?

	And only when such possibilities have been examined very closely and eliminated or found to be extremely unlikely would this be approved. We haven't finalized our plans for that flyby yet.

Man:	Well, what if you had to change it to like 100 kilometers, would that affect the tour at all?

John Spencer:	Yeah, it all affects the tour at some level. Or you can spend - you can spend fuel to deviate - pull yourself out and then push yourself back on to the tour later.

	But it's, yeah, it always has complicated ramifications either in having to adjust for the other things and move orbits down the line or having spent fuel to get back onto your original plan.

Man:	Yeah. What I mean is that you wouldn't have to change the tour.

John Spencer:	No.

Man:	Okay.

John Spencer:	No, it would be a small perturbation, it would be a hassle.

Man:	Okay.

John Spencer:	But a manageable hassle.

	But we do have to decide pretty soon. And, well, we'll hopefully learn more today from these new pictures of the plume, get some better idea of the density of the particles from today's observations under other things that will add to that. I forget when the final decision will be made. Amanda, do you know?

Amanda Hendrix:	About the trajectory for…

John Spencer:	The trajectory for the Rev 61 flyby.

	In the fall I think.

Amanda Hendrix:	I guess.

John Spencer:	Yeah, yeah.

	But yeah, you don't fly that close very lightly. You put a lot of thought into it.

Woman:	John, I think I missed this when you were talking about it earlier. The oxygen event that you just observed on (in-down) to Saturn, the one whether, you know, on Christmas they saw there's sort of this - they're doing one of those system scans and then by the time of July when they actually got into orbit it was gone.

	Is that - I know - associated I'm sure with Enceladus, is it - is there any more on that that I've missed recently?

John Spencer:	I sure don't know much more about that. Yeah, there was evidence that the oxygen density varied with time during the approach - there were period of many months when Cassini was approaching Saturn before we went into orbit where you've had a chance for a good long look at pretty similar geometry coming in.

	And yeah, there was evidence that the oxygen amount varied quite a lot.

Woman:	I mean, are they thinking, you know, sort of some spectacular eruption that lasted a short period of time and that it was over or…

John Spencer:	I honestly don't know. Things like that, you're also having to worry about the - how the magnetosphere is varying. Are we seeing variations in the amount of gas being produced? Are we seeing variations in how long the gas lives due to whatever process is our ionizing it and destroying it? Are we seeing variations in how brightly it is lit up by the magnetosphere?

	And it's certainly not my field, so I haven't heard that there is a definite resolution of that. I think the evidence is - points to that there probably is some change in the amount of gas coming off, but I don't know how dramatically it changes.

	But that's another uncertainty if we decide that it's safe to flyby to Enceladus under normal conditions based on the evidence we have now and then there is some dramatic change right when we're flying past, we could get into trouble. And so we're trying to get some handle too on what the variability is with time.

Woman:	Okay, thanks.

John Spencer:	Any other questions?

Man:	Yeah, John, I have a question.

	Giving presentations to kids or adult groups, is it okay if we use one or two of the slides that are in this presentation?

John Spencer:	I have no problem with that. I mean it's posted on a public Web site, and so pretty much anything that goes on the Cassini Web site is fair game.

Man:	Well, this one actually was password-protected, so that's kind of why I asked.

John Spencer:	Okay. Well, maybe Amanda can comment on that. I personally have no problem with that.

Amanda Hendrix:	Yeah. I think that's the idea of the PowerPoint presentation, is that you all can - are allowed to use the information in your public talk. And that's why (unintelligible) are encouraged to present stuff that, you know, they're comfortable with definitely having out there. And most of the stuff has been either published or presented at a scientific meeting.

Man:	Okay. Thanks, Amanda. Thanks, John.

(John Smith):	This is (John Smith).

	Is (unintelligible) PowerPoint format as well?

Amanda Hendrix:	Yeah, that's on the password-protected site, (John).

(John Smith):	Okay.

Amanda Hendrix:	And the details on how to get there is in the invitation, the email invitation that goes out.

(John Smith):	All right, thank you.

Amanda Hendrix:	Okay.

	Any other questions for John Spencer?

Man:	Yes, I have a question.

Man:	Just to say thank you, John.

Man:	Can you hear me?

John Spencer:	I certainly can, yeah.

Man:	On this projected lower altitude (unintelligible) kilometers, how fast will the Cassini be traveling?

John Spencer:	Oh let's see, that's in the table on Slide 57, because that gives the - yeah, the closest ones currently planned are on Orbit 80 and 88 which are 21-kilometer altitude and nearly 18 kilometers per second.

	So, you know, the distance you're traveling one second is equal to the distance between you and the surface. Of course you're traveling parallel to the surface and not towards it. So if you are on board the spacecraft, you will - pretty spectacular view of the surface, you know, rushing past you at a pretty furious rate.

	And we won't be taking pictures from that altitude. They'd be just completely blurred because we are going so fast. The idea of going that close is more to investigate the gas and dust environment and not to take pictures in that range.

Man:	Thanks.

John Spencer:	Okay. Anymore questions?

(Robert Marble):	Yes. (Robert Marble) from California. Do you hear me okay?

John Spencer:	Loud and clear.

(Robert Marble):	Great, great. Hey, good talk.

	First off, when presenting to the general public about this, what would you say is the most exciting thing during the Cassini observations of Enceladus? What has been the most exciting to you personally?

John Spencer:	I know the whole thing has been pretty thrilling. I remember a series of telecoms we had in the days after the July 2005 flyby and it was just amazing that I've been looking at our data and I saw this really weird hotspot of the South Pole and then Candy Hansen had been looking at the ultraviolet data and she was seeing the stellar occultation by gas over the South Pole.

	And the images were so strange and just the way everything came together in the period of a few days, just we're all comparing notes, "Hey, we see this." "Oh, we see this too." "Hey, we see it too." And we're all looking in different ways but seeing the same thing. Was just really thrilling because we just have never seen anything like this before except on Io where we see the volcanic activity and we - it's a very different environment, that sort of rocky world with high temperature volcanoes, and those discoveries were made like when I wasn't even in this business and I wasn't part of them.

	So to actually be part of a discovery like this was, with, you know, the second moon in the outer solar system that we have proved to be geologically active and that we've learned so much about it in such a short period of time has been very thrilling.

	But in terms of the general public, I think the thing that is most potentially significant is the here is an environment with probably liquid water and an environment where we can imagine that life might have been (if at all). And I think this is what will capture the public imagination more than anything else.

(Robert Marble):	And was there anything that was what you predicted, in other words, for the students that like to make hypotheses and then prove whether they're correct or not, was there things you predicted about Enceladus that were dead spot on?

John Spencer:	Well, yeah. I mean you'd go back to that National Geographic painting from 1981 that I showed near the beginning of the talk, and that's not what the (guise) of an Enceladus looked like. The artist who did that hadn't been thinking about the fact that the gravity is very low on Enceladus, so instead of getting a jet of water or gas that comes up and falls back to the surface, it just keeps going and it leads to the surface entirely.

	That, apart from that little detail, I mean that I think is a pretty remarkable example of how we got some very scattered clues 25, 30 years ago and there were a lot of ideas kicked around, but some of those ideas were spot on.

((Crosstalk))

(Robert Marble):	…now what was totally different - what was totally different than what you expected?

John Spencer:	Well…

Woman:	Localized at the South Pole?

John Spencer:	Well, yeah, that's a good point. That's a very strange thing that no one would have predicted.

	And to me, I'm still really puzzled by the fact that we have this extreme contrast in ages. We talked about Europa. And the whole surface of Europa seems to be about the same age from the number of craters, it's, you know, tens of millions of years old. Presumably some areas are older than others.

	But the older areas that we see on Enceladus are much more heavily cratered than anywhere on Europa. So, from our estimates of how fast things are heating the surface of Enceladus, we'd guess they may be a billion years old or more.

	But then we have other areas with no craters at all that are less than a million years old that are active right now. And so we have thousand-fold variation in the age of the surface. Some areas have done nothing for (unintelligible) are being reworked right the second. I still have trouble getting my head around that.

	For me I think this is one reason, well, as I mentioned, this is the main reason I didn't believe in (unintelligible) on Enceladus earlier, and I just can see how you could have such old terrain next to such young terrain…

((Crosstalk))

John Spencer:	…figure out the to do it.

(Robert Marble):	Amazing.

	And one last question. About the rings of Saturn, they're so well associated with astronomy in general and Saturn in particular. There's been a lot of talk about how long they've been around and how long they can sustain themselves. Does this give any new insights into the ability of the rings to sustain themselves?

John Spencer:	Not directly. The E ring that Enceladus creates is a very different ring from the main ring. It's very fine dust. It doesn't last very long.

	The main rings produced - made more boulders than of dust. And they are, you know, completely different.

	The ways in which Enceladus may help us to understand that is by understanding Enceladus we get a handle on what the title forces are, what effects Saturn has on the orbits of objects around it which in turn telling you something about the internal structure of Saturn. And that in turn can feed back into the evolution of the rings.

	And I don't think anyone's made a direct connections that, oh, because Enceladus is doing this, it means that about the rings yet. But it helps put that story together because it's constraining how Saturn affects the orbits, things around both moons and ring particles.

(Robert Marble):	Fantastical, great presentation. Thank you very much.

John Spencer:	Well, thanks for stopping by.

	Anyone else has questions?

Man:	Yes, one question please.

John Spencer:	Sure.

Man:	Again, the dichotomy of the surface. The primary mechanism you think for the younger surface is probably ice movement rather than internal heating or snowfall?

John Spencer:	Well, not snowfall because that's a secondary process, that's coming out of the activity somewhere else. But it's - well, it's both the tidal heating is producing weak zones and then is producing the stresses that produce motion along the fractures. And so it's all interconnected.

	And I suspect that the answer does lie in the fact that once part of Enceladus is weak, the heat, the stresses are focused there and so it tends to stay weak and stay warm.

	And so you get this extreme localization of the activity. The areas that are heavily cratered I guess have a thick old cold crust and they just haven't had a chance to participate in this because they never get warm enough for the process to get started.

Man:	All right, thank you. And again, I enjoyed your presentation as well.

John Spencer:	Thank you.

	Any more questions?

Amanda Hendrix:	Okay. Well, thanks again, John, so much. It was a great talk.

John Spencer:	Thank you, I enjoyed it. Thank you all for stopping by.

((Crosstalk))

Amanda Hendrix:	…and the next CHARM telecon is on May 29 and the topic will be Saturn's aurora, so we hope to hear from all of you again then.

	Thanks again, everybody.

John Spencer:	Thanks, Amanda. Bye-bye.

Amanda Hendrix:	Bye.

Woman:	Thanks.

END